In the human body, two different types of sub-units occur in this enzyme, namely sub-units M and B. Since the active enzyme is composed of two sub-units, and since the two sub-units can combine freely with one another, three types of enzyme are possible: the muscle type CK-MM, the brain type CK-BB and the hybrid type CK-MB, which occurs mainly in the myocardium, passes into the serum in myocardial infarction, and can then be found in the serum in an elevated concentration. The activity of this hybrid isoenzyme, in addition to the total activity of the CK in the serum, can be used in the diagnosis of myocardial infarction.
It is known to add specific antibodies against the M sub-unit of CK so as to exclude the muscle type CK-MM of the isoenzyme, and then to measure the remaining hybrid enzyme CK-MB by any of the known methods for the determination of CK, using either precipitating or inhibiting antibodies. One disadvantage, however, is that in this case the hybrid enzyme CK-MB is 80% inhibited (Clin. Chim. Acta, 58, 223-232 (1975)). It was already known to obtain antibodies which completely inhibit the muscle type CK-MM, but which do not at all inhibit the brain type CK-BB (Immunochemistry, Vol. 6, 681-687 (1969)). Although in such antibodies the sub-unit B in the hybrid enzyme CK-MB is not at all inhibited and therefore the reliability of the method of determination is thereby increased, this method also has considerable disadvantages. For it has been found that, even when high-purity antigens (CK-MM) are used, the animals used for the formation of the antibody mostly supplied a serum which effected a more than 55% inhibition, usually between 60 and 90%, of the hybrid enzyme CK-MB. Therefore, it was necessary, for each individual animal used for the immunization, to find out whether the antibody that had been formed inhibited the CK-MB to only 50%, or inhibited it more strongly. Aside from the expense and difficulty this involved, it was found that most of the serums obtained were virtually unusable, since different degrees of inhibition always occurred. Even pooled sera are only a partial remedy for this difficulty. In addition, the amount of CK-MB in the serum in myocardial infarction is extremely small, and in the case of antibodies which exclude any appreciable additional amount thereof, the reliability of the method is greatly reduced.